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British and Pakistani officials stepped up an eight-month investigation to prevent a “dry run” of a terrorist attack to blow up more than 10 U.S. airplanes flying between the U.S. and the United Kingdom arresting 24 suspects.

The terrorist suspects had been under surveillance since December and the plot could have been carried out “within days,” officials said.

U.S. officials publicly congratulated Britain for the arrest, but privately two officials suggested that electronic surveillance of terror suspects in Pakistan and Britain provided the initial clues to the plot. Several news organizations, including ABC, Fox and CNN, reported last night that a British undercover agent was able to penetrate the terrorist cell.

British authorities decided to take action and arrest as many people linked to the plot as possible Wednesday before they could carry out the planned “dry run” to determine whether they could smuggle the contraband onboard, said officials who spoke on the condition of anonymity.

British security officials “decided that no way were they going to allow those guys to get on the planes, because what if they were not doing a dry run?” said one federal security official.

As of yesterday, 24 suspects had been arrested and more were thought to be at large. Officials declined to specify a number, but suggested that a plot to blow up 10 planes would likely involve about 50 terrorists.

The scheme sought to cause maximum psychological damage by using TATP — the material thought to have been used in the July 2005 attacks on London’s mass transit — to explode the planes in flight and make the recovery of bodies difficult.

“The psychological impact of not knowing where your loved ones are is devastating,” the official said. “The goal was to curtail international commerce because people would be so scared to fly internationally.”

The introduction of TATP (triacetone triperoxide) to the aviation arena is a new development in the terrorist arsenal, the official said.

“It could cause catastrophic failure. Rest assured, these guys did their homework on where to place it,” the official said.

The scheme to blow up airplanes with liquid explosives and electronic detonators was international in scope, well planned with a significant number of operators and “suggestive of an al Qaeda plot,” Homeland Security Secretary Michael Chertoff said yesterday.

The London-based terrorists planned to use containers of sports drink with compartments full of liquid or gel explosives that were to be triggered by the flash of a camera, said officials familiar with details of the operation.

Officials said planning outlines resembled the al Qaeda plot developed in the Philippines in 1994 to blow up 11 U.S.-bound jetliners using hidden liquid explosives. That plot was hatched by Ramzi Yousef and Khalid Shaikh Mohammed, al Qaeda leaders now held in prison.

“Given the amount of planning and effort that was put into this plot, I think it would be a little bit risky to assume that everything is shut down and the threat has gone away,” Mr. Chertoff said during a press conference after the scheme had been disrupted with police raids that began Wednesday night in Britain and later in Pakistan.

“This was a very sophisticated plan and operation,” Mr. Chertoff said. “The conception, the large number of people involved, the sophisticated design of the devices that were being considered, and the sophisticated nature of the plan all suggests that this group that came together to conspire was very determined and very skilled and very capable.”

His deputy, Michael Jackson, said U.S. officials knew the arrests were imminent for several days, but waited for Britain to take the lead.

Unlike the September 11 attack, the latest scheme involved a mixture of both Pakistani-based Islamists and “homegrown” terrorists who had been recruited in Britain’s large Muslim community, the officials said. The September 11 plot involved mainly Saudis who were recruited to conduct suicide aircraft attacks after seizing control of the jets. That plan involved five terrorists per plane.

Officials confirmed that three of the terror plot ringleaders are Rashid Rauf, Mohammed al-Ghandra and Ahmed al Khan, whose identities were first disclosed by ABC News. The men had been traced to Pakistan, which has been a major base of operations for al Qaeda.

Officials said the Pakistani government is cooperating in the investigation, since several of the key planners linked to the plot received financial or logistical support from al Qaeda-linked terrorists in that country. Three Pakistanis were arrested in Islamabad and Lahore, the Associated Press reported from Islamabad, quoting Pakistani officials. Officials yesterday were reluctant to say the danger has passed.

“It would be premature to declare it a complete success,” the senior official said. “We’re still very concerned.”